1. Field of the Invention
The present invention relates to a flat display device, and more particularly, to an LCD panel with integrated driving circuits.
2. Discussion of the Related Art
As the information society develops, demands for various types of display devices increase. Accordingly, many efforts have been made to research and develop various flat display devices such as liquid crystal display (LCD), plasma display panel (PDP), electroluminescent display (ELD), and vacuum fluorescent display (VFD), and some species of the flat display devices are already applied to display devices of various equipments. Among various flat display devices, the LCD device has been used most widely as a mobile display while replacing a CRT (Cathode Ray Tube) because of its advantages, such as excellent picture quality, lightweight, thin profile, and low power consumption. In addition to the mobile type LCD device, such as a display for a notebook computer, the LCD device has been developed for computer monitors and televisions.
The LCD device includes two substrates facing each other with a liquid crystal injected in between the two substrates. The liquid crystal has a different alignment direction according to temperature or density of the liquid crystal. Liquid crystal has both the fluidity of a liquid material and crystalline characteristics of a solid material. That is, a liquid crystal lies between the solid and the liquid phase, which is like a state just before a melting solid becomes a liquid. When a light is irradiated onto the liquid crystal, or an electric or magnetic field is applied to the liquid crystal, the liquid crystal has the birefringence of an optical anisotropic crystal. Also, the liquid crystal has the characteristics of both the liquid and solid within a predetermined temperature range.
FIG. 1 is a perspective view illustrating a related art LCD device including a liquid crystal display (LCD) panel for displaying an image from a driving circuit that generates a video signal. As shown in FIG. 1, the LCD device includes a plurality of gate lines 14 and data lines 16 that are formed on a first substrate 21 in a matrix. Thin film transistors 13 are formed where the gate lines 14 and data lines cross over each other. Then, a color filter with a black matrix 23 and common electrode 24 are sequentially formed on a side of a second substrate 22 that opposes the first substrate 21. A liquid crystal 25 is positioned between the first and second substrates 21 and 22. The common electrode 24 faces pixel electrodes 26 formed on the first substrate 21 such that each of the pixel electrodes together with the liquid crystal 25 and the common electrode 24 define a pixel of the related art LCD device.
As shown in FIG. 2, each of the thin film transistors 13 includes a gate electrode 30 made of a metal, such as aluminum Al, chrome Cr, or molybdenum Mo. Further, each of the thin film transistors 13 includes source electrode 32 and drain electrode 33 made of a metal, such as aluminum Al, chrome Cr, or molybdenum Mo. A semiconductor layer 34 is positioned in between the source electrode 32 and the drain electrodes 33. Impurity semiconductor layers 36 are respectively provided between the semiconductor layer 34, and the source electrode 32 and drain electrodes 33. The gate electrode 30 is connected to the gate line 14 shown in FIG. 1, the source electrode 32 is connected to the data line 16 shown in FIG. 1, and the drain electrode 33 is connected to the pixel electrode 26. When a scanning voltage is applied to the gate electrode 30 through the gate line 14, the thin film transistor is operated so as to apply a data voltage from the data line 16 to the pixel electrode 26.
The data voltage applied to the pixel electrode 26 through the thin film transistor generates a voltage difference between the pixel electrode 26 and the common electrode 24. The voltage difference changes the arrangement of the liquid crystal 25 between the pixel electrode 26 and the common electrode 24 such that light transmittance characteristics of the pixel are changed. Thus, a visual image can be created with the pixels of an LCD device according to data voltages applied to the pixels.
Recently, the trend in LCD devices has been higher resolutions at larger sizes. Further, efforts have been made to manufacture the active region display part and the driving circuits for the display part on the same substrate. The driving circuits should use thin film transistors having a high mobility as switching device for high speed switching of video signals. However, the thin film transistor (TFT) in driving circuits formed on the same substrate of the display part in the related art includes the same amorphous hydride silicon (a-Si:H) with low mobility as the semiconductor layer 34 of the TFTs of the pixels. Thus, the related art LCD device has problems in achieving higher resolutions at larger sizes that require high speed switching of video signals.
A method for overcoming the switching problem is to use a polycrystalline-silicon TFT (Poly-Si TFT) instead of amorphous hydride silicon TFTs. More particularly, the polycrystalline-silicon TFTs are used in the driving circuits, which are integrated into the LCD panel. Also, active layers of the TFTs in the pixel, such as the semiconductor layer 34 in FIG. 2, are made of polycrystalline silicon, thereby preventing image blur due to slow switching in the pixels. Furthermore, because a process step, such as of Chip On Glass (COG) or Tape Automated Bonding (TAB), for connecting the driving circuit (driver IC) to the LCD panel does not have to be performed since driving circuits are formed on the same substrate as the active region, manufacturing costs are decreased.
FIG. 3 is a cross-sectional view illustrating a related art LCD panel using polycrystalline-silicon TFTs. As shown in FIG. 3, the LCD panel of the related art LCD panel includes an active region 1 for displaying an image, a data driving circuit 2, a gate driving circuit 3, and an input terminal 4. The data driving circuit 2 is integrated into the LCD panel for driving data lines 5 in the active region 1. The gate driving circuit 3 is integrated into the LCD panel for driving gate lines 6 in the active region 1. The input terminal 4 is integrated into the LCD panel for providing a data signal to charge the data lines 5, a first power signal to operate the gate driving circuit 3, a second power signal to operate the data driving circuit 2, an electrostatic discharge protection circuit, and a control signal to control the gate driving circuit 3 and the data driving circuit 2. The data driving circuit 2 is separated from the gate driving circuit 3 such that the data driving circuit 2 applies signals to the data lines 5 and the gate driving circuit 3 applies signals to the gate lines 6.
As shown in FIG. 4, data lines 45 are divided into blocks, which each receive video signals from the plurality of signal lines S1–Sn through a plurality of switching devices 46. Each of the blocks includes a shift register 43 and a buffer 44 for driving data lines 45 divided into “m” blocks. The plurality of signal lines S1–Sn are formed so as to transmit a video signal output from a digital-to-analog converter (not shown) to each of the data lines 45. The plurality of switching devices 46 sequentially applies the video signals of the signal lines S1–Sn to the data lines for each block according to driving signals output from the shift registers 43 and the buffers 44. It is possible to drive the data lines 45 divided into “m” blocks since the active layers of the TFTs in the pixels are made of polycrystalline silicon having a high electron mobility.
The driving circuits of the related art LCD panel using polycrystalline silicon TFTs is different from a related art driving circuits using amorphous hydride silicon TFTs in that the respective data lines are divided into “m” blocks so as to decrease the number of contact lines between the panel and an external circuit during selecting the gate line, whereby a display voltage is sequentially provided to the data line. The related art LCD panel using polycrystalline silicon TFTs has other advantages. For example, if the LCD panel is a reflecting type LCD panel, the LCD panel includes multi-layered metals for the gate electrode, the source/drain electrodes, a reflecting electrode and a pixel electrode, and an insulating layer.
The plurality of signal lines S1–Sn of FIG. 4 is single layer structure that can be formed during the metallization process for forming either the gate electrodes, source/drain electrodes or pixel electrodes. As shown in FIG. 5A and FIG. 5B, a plurality of signal lines S1–Sn formed in a single layer requires the use of an area of the related art LCD device that cannot be used as an active region. The unused or non-active area that cannot be used in the active region increases in proportion to the number n of the signal lines S1–Sn.